1. Field of the Invention
The present invention relates to a process for preparing a water-dispersion varnish for electrodeposition, and more particularly, it relates to a process for preparing a water-dispersion varnish for electrodeposition which comprises a modified polyesterimide having ester groups and imide groups in the main chain or a modified polyesteramideimide having an ester group, an imide group and an amide group in the main chain.
2. Description of the Prior Art
Electrodeposition of resin coatings onto conductive substances is commonly used in many technologies. The electrodeposition coating method has several advantages: it is safe and hygienic because it employs no organic solvent; it can form a film of uniform thickness in a short period of time; and it is easily automated as compared with conventional spray coating or dip coating methods.
In the usual instance, a varnish is used in the electrodeposition process, wherein a coatable polymeric composition is either dissolved into an electrolyte medium or is dispersed in fine particles having diameters of 0.1.mu.. When a solution varnish is used, uniform deposition can be attained even in a thin layer. However, the thickness of the film may only be as large as several tens of microns. Accordingly, solution varnish is not suitable if a thickness larger than 100.mu. is required. Moreover, most water soluble or water insoluble polymers used for the solution varnish will contain many easily dissociated groups so that a coating employing such polymers would not be a good electrical insulator.
On the other hand, a dispersion varnish is inferior to a solution varnish from the viewpoint of uniformity of film thickness. It is difficult to form such a uniform film on a substrate having a complicated shape. However, dispersion varnish has the advantages in that a thick film can be formed easily in a short period of time and the durability of the finished film can be improved by using a polymer having a high molecular weight. Moreover, the polymer used in the dispersion varnish is not easily dissociated and, accordingly, a film containing it will have good electrical insulation properties.
A need therefore exists for a water-dispersion varnish that is capable of providing an electrodeposition coating of good uniformity. Such a varnish would be expected to find a great deal of acceptance in a wide variety of fields.
There are two methods for preparation of a water-dispersion polymer: by emulsion-polymerization using monomers having an unsaturated group; or by dispersing a polymer by suitable processes. The monomers used for the former method are limited to vinyl compounds and accordingly, the characteristics of the finished films are limited. In the latter method, good electrical insulation properties of the finished films can be produced by employing epoxy resin, polyester resin, formal resin, etc. However, it is quite difficult to prepare a water-dispersion varnish of the latter type which is suitable for electrodeposition. Accordingly, this type of varnish has not been commercialized.
Recently, polyesterimides and polyesteramideimides have been found to possess excellent electrical insulation characteristics and excellent thermal stability. They are quite economical and hence have been widely used. It would be most desirable to prepare a water-dispersion varnish which is suitable for electrodeposition by using such resins.